Suitably shaped thermoplastic polymers have been routinely used as packaging for products that degrade under normal environmental conditions. Thermoplastic polymer packaging provides a passive barrier to diffusive mass transport of oxygen, other atmospheric gases, water vapor, ethylene, and flavor and odor compounds through the packaging walls. Passive barriers are defined as being chemically inert to the permeating gasses. In comparison to metals and inorganic glasses, which provide nearly absolute gas barriers, thermoplastic polymers are more permeable to atmospheric gasses including oxygen, and such polymer barriers often do not provide sufficiently low oxygen transmission rates to satisfy manufacturers' expectations of product shelf life. In addition, passive barriers do not reduce oxygen initially present inside the package after sealing. Oxygen present inside the package or permeating through the packaging walls can accelerate oxidative, enzymatic, and other adverse biochemical reactions. Accordingly, many food and beverage manufacturers use preservatives and antioxidants in a packaged foodstuff to extend its shelf life. Consumers, however, increasingly find such additives undesirable.
Oxygen-absorbing materials have been placed inside the packaging to reduce oxygen levels inside sealed packages. The oxygen-absorbing materials can be arranged as separate packets (also known as a pouches or sachets) or as labels attached to the inside of the packages. Although oxygen-absorbing materials placed inside the packaging can reduce the amount of oxygen inside the packaging, the rate of oxygen ingress into the packaging through the packaging walls is not slowed by the presence of oxygen-absorbing materials within the packages.
However, oxygen-absorbing materials have also been incorporated into the polymer resin of the packaging walls to form active barriers to oxygen ingress. The polymer resin in the walls can be impregnated with so-called oxygen scavengers containing oxidizable substrates that undergo irreversible chemical reactions with oxygen. For example, the oxygen-scavenger particles can be manufactured as fine powders (particulates) that can be dispersed in a liquid polymer resin during the melt processing. The scavenger-loaded resin can then be processed into a packaging film or a container wall. The oxygen-scavenger particles incorporated into packaging walls form an active barrier to oxygen permeation that reduces rates of oxygen ingress into the package. In addition, the incorporated oxygen-scavenger particles can reduce both the amount of oxygen dissolved in the packaging walls and the amount of oxygen present inside the package.
Oxygen-scavenging powders can be formed from a mixture of an oxidizable metal (e.g., reduced iron powder) and an activating component promoting the reaction of the oxidizable metal with oxygen, often in the presence of water. Suitable activating components include: electrolytes (e.g., sodium chloride), acidifying components (e.g., ferrous sulfate), electrolytic acidifying components (e.g., sodium bisulfate), and protic solvent hydrolyzable halogen compounds (e.g., Lewis acids, such as aluminum chloride).
Reducing average particle size and mixing the oxidizable and activating components, e.g., using grinding and dry blending, are known to improve scavenging performance of the compositions. However, such methods also result in significant amounts of loose activating component particles lacking intimate contact with the oxidizable substrate. Such loose particles can act as nucleation centers promoting crystallization of semi-crystalline wall forming matrix polymers (such as HDPE, PP and PET). In addition, the random separation of the oxidizable components from the activating components reduces the efficiency of the oxidation reactions. As a consequence, high loadings of the oxidizable and activating components within the polymer resins may be needed to achieve the desired low oxygen transmission rate and high reactive capacity of the active barrier. The large number of loose particles, irregular particle morphologies, and the nucleating effects and subsequent crystallizations of the matrix polymer often lead to increased haze and reduced clarity of such active barriers.
Consumers of packaged food and beverage products generally prefer clear transparent packaging that allows visual evaluation of the product quality attributes such as consistency, texture, and color before purchase. Oxygen-scavenging blends incorporated into the walls of clear plastic containers can produce a haze or color brought on by a large number of small discrete particles of different chemical nature that are capable of scattering visible light. US Patent Application Publication Nos. 2003/0027912, 2003/0040564, and 2003/0108702 all of Tung et al., which are hereby incorporated by reference, discuss limiting the size and concentration of oxygen-absorbing particles to reduce haze.
Combining oxidizable component particles with activating component particles into heterogeneous entities greatly improves the efficiency of the oxygen-scavenging compositions and allows for reducing the total number of particles that must be loaded into film-forming polymers. The reduction in the number of particles can be achieved by attaching smaller activating component particles to larger oxidizable component particles. By controlling the initial particle size distribution of the components and the component weight ratios, the heterogeneous particle sizes of oxygen-scavenging composition can be kept below a limit of approximately 30 microns to 50 microns to avoid visual detection of such compositions as dark spots in the transparent polymer resin barrier.
U.S. Pat. No. 5,744,056 to Venkateshwaran et al., which is hereby incorporated by reference, discloses oxygen-scavenging compositions that exhibit improved oxygen-absorption efficiency by including more than one type of activating component. For example, a preferred composition includes both an electrolytic activating component and a non-electrolytic acidifying component. In the presence of moisture, the combination of the electrolytic and the acidifying components promotes the reactivity of metal with oxygen to a greater extent than either component alone.
These conventional scavenging compositions are typically created by dry blending the ingredients or by depositing the electrolytic and acidifying agents onto metal particles out of an aqueous liquid or slurry. European Patent Application Publication No. 1,506,718 entitled “Oxygen-scavenging Compositions and the Application thereof in Packaging Containers” and International Patent Application Publication WO 2005/016762 entitled “Oxygen-scavenging compositions and the application thereof in packaging and containers” both of Cobarr S.p.A. provide for depositing certain protic solvent hydrolyzable activating components onto oxidizable metal particles by dissolving the activating component into an essentially moisture free organic solution, contacting the solution with the oxidizable metal particle, and then removing the solvent. While depositing activating compounds from a liquid phase can achieve the desired intimacy of contact for unitary particles, liquid phase deposition presents several problems. First, the deposition leaves behind impurities of the solvent or reaction products of the salt with the solvent, referred to as adducts. These impurities can be bound into the composition. Second, the liquid phase deposition requires a dissolution step and a solvent removal step. Third, surface tension of the liquid can inhibit penetration of the liquid into the pores of metal particles. Fourth, the composition tends to be unstable during subsequent heat processing of the active barrier polymer.
Vapor streams have also been used to achieve intimate contact between oxidizable carrier components and activating guest components. For example, Japanese Application 10-131379 entitled “Iron Powder For Reactive Material and Its Production” provides for enveloping an iron powder with hot chlorine or hydrogen chloride gas for forming a ferric chloride coating on the surfaces of the iron powder. Although this vapor phase-solid phase reaction creates intimacy of contact, the reactions are limited to the reaction products of iron and a few gasses. Dissimilar components such as iron and salts of alkali and alkali earth metals cannot be combined by this technique.
Other processes involving sublimation and physical vapor deposition have also been used to combine oxidizable and activating components into heterogeneous oxygen-scavenging particles. For example, certain Lewis acids such as AlCl3 and AlBr3 can be vapor deposited onto iron particles. However, these techniques are limited to activating components with relatively low sublimation temperatures that allow for efficient vaporization and delivery of the activating component vapor to a continuously mixed iron powder without decomposing at these temperatures. Activating compositions that contain more than one activating component generally cannot be deposited in this way because of differences in sublimation temperatures of the components. Some components have excessively high temperatures of sublimation, and other components decompose before sublimation temperatures are reached. Unequal sublimation rates can also alter the final composition.